I have made an image of a CD-ROM containing 648 Mb of data. I have tried both the .ccd and the .mds file formats, but both of them results in files exceeding 750 Mb in size. What happens, and is there a way to bring down the file size?

In each case a raw image has been made (which is what you want). A raw image is ~19% larger than the space it will occupy when burnt. When you burn the image back to a cdr/w it will only occupy 648mb just like on the original.

What about other image formats then? Using the .iso format, the result is an image of roughly the same size as the original disc. So, is the .iso format technically inferior to the .mds format? Or does the iso format simply allow for compression to take place?